The single largest cause of cardiovascular disease is sclerosis ----a build-up of fatty or calcific deposits in the arterial lumen. These deposits can impair, and in severe cases, totally obstruct, i.e., become an occlusion, the flow of blood through the artery. A number of medical devices have been designed to displace, disperse or extract the occlusive deposits. Most of these devices operate over or in conjunction with a guide wire used to navigate the vasculature and traverse the occlusion location. The initial placement of the guide wire can be problematic in cases of total or near-total occlusion. Known techniques for traversing a total occlusion involve forcibly advancing a blunt catheter through the occlusive material (the Dotter technique), or using rotational means (orthogonal displacement of friction).
A more recent development involves using an RF-activated guide wire to electrosurgically recanalize the occlusive material, the details of which are described in U.S. Pat. No. 5,364,393 issued to Auth et al., which is fully incorporated herein by reference for all that it discloses and teaches. As taught in Auth, an electrically conductive guide wire is proximally connected to a radio frequency (RF) generator, which when operated transmits RF energy through the guide wire to a spherical ablation tip.
More particularly, the guide wire is first advanced through the vasculature of a patient, via a guiding catheter or guide sheath, until it reaches the occlusive material. With the spherical ablation tip in contact with the occlusive material, the RF generator is operated and the guide wire tip is advanced through the occlusive material. A therapeutic device used to treat the occlusive disorder is then advanced over the guide wire in accordance with known techniques.
Although the recanalization technique taught by Auth is generally effective, it is desirable to improve the devices and methods used in this approach to provide a more efficient and safe treatment of sclerosis caused by total or near-total occlusions within the arterial vasculature.
In particular, to remove or ablate the occlusive tissue matter quickly and effectively, the electrode tip of the ablative guide wire must be supplied with a potential high enough to ionize or break down the liquid contained in the tissue. This is known as a "spark erosion" process. With a monopolar guide wire electrode tip used in conjunction with a dispersive electrode or ground pad located on an external portion of the patient's body, an ionizing arc from the electrode tip is used to instantaneously convert the occlusive matter into a plasma state, in effect, vaporizing the tissue into particulate matter that is safely absorbed by the blood stream. Once the spark erosion process is initiated, a lower energy potential may be employed to maintain the plasma conversion as the guide wire tip is moved through the occlusive matter.
Towards this end, the RF generator system must provide the guide wire electrode tip with sufficient voltage or potential to initiate the spark erosion process. Typical RF generators, such as those used in electrosurgery or electrophysiology, are capable of generating a high potential, but deliver a constant output level under all load impedance conditions. Within the body, however, the load impedance seen by the guide wire electrode tip may vary greatly, depending upon the relative liquid content of the body tissue it contacts, i.e., the lower the liquid content, the higher the impedance. For example, blood impedance will typically range from 150 to 200 ohm/cm. Healthy vessel wall impedance will typically range from 300 to 400 ohm/cm. Occlusive tissue impedance, on the other hand, depending on the degree of calcification, will normally exceed 600 ohm/cm, ranging from 1000 ohm/cm to as high as 3000 ohm/cm.
Because it is difficult to determine the exact position of the guide wire electrode tip within an occluded vessel, producing a sufficiently high potential to initiate the spark erosion process can be problematic. In particular, when in contact with relatively low impedance blood or healthy vessel wall tissue, a sufficient potential is difficult to achieve without increasing the output power to a level that may cause damage to tissue remote from the surgical site, e.g., in the form of unwanted charring or ablation of healthy tissue. The increased power may also pose risk of a dangerous electrical shock to the attending surgeon, as well as loss of control sensitivity.
U.S. Pat. No. 5,300,068("Rosar") discloses an RF generator system for selectively providing a train of modulated electrical energy pulses in a modulated continuous wave signal (preferably a cosine squared wave shape) to an electrosurgical electrode disposed on a guide wire, wherein the output impedance of the source of the pulses is continually matched to the load impedance seen by the electrode. In particular, the Rosar generator system measures the relative electrical energy produced by an arc in response to a given electrical pulse, and compares the relative electrical energy to a predetermined value to determine an energy difference. The energy level of a subsequent pulse is then adjusted to reduce the measured difference towards a pre-selected value. According to the Rosar patent, this automatic impedance matching compensates for the changing impedance conditions at the electrode, to ensure an efficient power transfer takes place. In particular, maximum power transfer will occur if the output impedance at the electrode tip is substantially equal to the load impedance of the body tissue (or blood) in contact with the electrode tip.
However, the Rosar generator system is relatively complex and, thus, expensive to implement. Further, because the ablation electrode power output is maximized over all impedance levels, overheating of the electrode in the blood pool or when in contact with healthy vessel wall tissue may result, thus damaging the electrode structure and potentially harming the patient.
Thus, it would be desirable to provide a simplified RF generator system for providing energy pulses to the electrode tip of an ablation guide wire of a voltage potential sufficient to initiate the spark erosion process when in contact with relatively high impedance occlusive tissue, but which will minimize power output when in contact with relatively low impedance blood or healthy vessel wall tissue.